Darkness and Light

Most of the Universe is made of stuff we can't see, so-called "dark
matter". This material can only be detected by the effect of its gravity on
nearby "visible" matter. The images on the right show optical images of 2
clusters of galaxies. The visible light is dominated by the combined
starlight from the individual stars in the member galaxies (along with bright
arcs produced by gravitational lensing of distant objects by the mass in
the galaxy cluster). The images on the left are X-ray images of the same
clusters obtained by the Chandra X-ray
observatory. the X-ray images trace the distribution of the dark
matter. If our eyes were sensitive to dark matter, the galaxy clusters
would appear similar to their appearance in the X-ray images. In most
clusters of galaxies, most of the mass is comprised of "dark matter"; only
a small percentage of the matter in these clusters is visible.